Multiple input multiple output (MIMO) wireless communications systems include multiple antennae. Spectrally efficient techniques are employed to increase data rates over wireless channels, which typically have limited bandwidth and transmit power. Space-time processing techniques are commonly used to increase spectral efficiency and data throughput per channel bandwidth, which is commonly measured in bits per second per Hertz. For example, techniques such as adaptive array processing, spatial multiplexing, and space-time coding are employed to increase spectral efficiency and the reliability of data that is wirelessly transmitted in a fading environment.
There is interest in applying space-time processing techniques such as spatial multiplexing and space-time block coding to next generation wireless local area networks (WLANs). For example, the IEEE 802.11 high-throughput study group (HTSG) has proposed systems that have throughputs in excess of 100 Mbps. This requires a data rate that is greater than 150 Mbps to account for overhead from medium access control (MAC) device headers.
Referring to FIG. 1, a MIMO wireless communications system 10 includes a wireless communications device 11 and a wireless communications device 12. The wireless communications device 11 includes a radio frequency (RF) transceiver 13 with M antennae 14. The wireless communications device 12 includes a RF transceiver 15 with N antennae 18. The wireless communications device 11 also includes a space-time processor 20. An input of the space-time processor 20 receives a symbol sequence b={b0, b1, b2, . . . , bk-1} with k symbols. The space-time processor 20 formats the symbols for transmission and feeds the symbols to the RF transceiver 13. Symbols are transmitted by the M antennae 14 during one or more symbol periods.
In one configuration, the space-time processor 20 implements spatial multiplexing. Spatial multiplexing ideally produces an M-fold increase in system capacity (in bits per second per Hertz), where the RF transceiver 13 includes the M antennae 14 and the RF transceiver 15 includes the N antennae 18, and where N is greater than or equal to M. For example, with first and second antennae at RF transceiver 13, the first antenna transmits symbol c1 and the second antenna transmits symbol c2 during a first symbol period. During a second symbol period, the first antenna transmits symbol c3 and the second antenna transmits symbol c4. This approach requires that the system operates in a rich-scattering environment and that transfer functions between pairs of antennae at the wireless communications devices 11 and 12, respectively, are uncorrelated and may be separated by the wireless communications device 12. This is conceptually equivalent to transmitting data across M independent channels.
In another configuration, the space-time processor 20 implements space-time block coding. A space-time block code generates blocks that include one or more symbols. For example, the space-time processor 20 may implement a rate-1 orthogonal space-time code that encodes two symbols per block. With first and second antennae at RF transceiver 13, two symbols are transmitted during two consecutive symbol periods. For example, during a first symbol period, the first antenna transmits c1 and the second antenna transmits c2. During a second symbol period, the first antenna transmits −c2* and the second antenna transmits c1*, where c1* and c2* are the complex conjugates of c1 and c2, respectively. The space-time processor 20 transmits complex conjugates of the symbols to add redundancy and to allow a wireless communications device to reconstruct a signal in the event that a transmission path experiences noise and fading.
The N antennae 18 of the RF transceiver 15 receive signal transmissions through hij, illustrated at 22, where hij is the channel estimation of the channel between antenna i of RF transceiver 13 and antenna j of RF transceiver 15 during a symbol period. The wireless communications device 12 includes a space-time combination module 24. The RF transceiver 15 sends received symbols to an input of the space-time combination module 24.
The space-time combination module 24 outputs decoded data sequence {circumflex over (b)}={{circumflex over (b)}0, {circumflex over (b)}1, {circumflex over (b)}2, . . . , {circumflex over (b)}k-1} based on the received symbols. The space-time combination module 24 employs combining techniques such as zero-forcing or minimum mean square error (MMSE) techniques. With MMSE, received symbols are linearly combined using a set of weights that yields a minimum mean square error between the estimated sequence and the true sequence. Non-linear techniques such as V-BLAST may also be employed.
V-BLAST utilizes a recursive procedure that sequentially detects different signal components from antennae of the RF transceiver 15 in an optimal order. Spatial multiplexing is preferred over space-time block coding due to the M-fold increase in throughput with the addition of M antennae at the wireless communications device 11. However, successful utilization of spatial multiplexing requires a wireless communications system that operates in a rich-scattering environment.
Referring now to FIG. 2, a first wireless communications system 32 operates in a rich-scattering environment. The first wireless communications system 32 includes a transceiver 34 with first and second antennae 36-1 and 36-2, respectively, and a remote transceiver 38 with an antenna 40. In a rich-scattering environment independent transmission paths 42 exist between pairs of antennae at the transceivers 34 and 38, and many reflections, illustrated at 44, occur. The transmission paths 42 are uncorrelated at the remote transceiver 38, and line of sight (LOS) does not exist between pairs of antennae at the transceivers 34 and 38.
Referring now to FIG. 3, in a second wireless communications system 46, LOS exists between the second antenna 36-2 at the transceiver 34 and the antenna 40 at the remote transceiver 38. The transmission paths 48-1, 48-2, 48-3, and 48-4 may be correlated at the remote transceiver 38 and/or the received signal at the remote transceiver 38 may be dominated by transmission path 48-3. This can complicate the separation of independent transmissions. Insufficient scattering and/or spacing between pairs of antennae at the transceivers 34 and 38 can cause fading to be correlated. Additionally, in a “keyhole” effect environment, fading is correlated when multiple transmissions merge and then diverge, which complicates signal separation at the remote transceiver 38. Space-time processing techniques are largely ineffective in LOS and “keyhole” effect environments as well as other situations that cause fading correlation.
Significant increases in transmit power are required to maintain a desired throughput while such environments exist. However, many wireless communications systems at given bandwidths are power-limited by regulatory bodies. Polarization and/or array geometries of antennae at the local and remote transceivers 34 and 38, respectively, may also be exploited by spreading out antenna elements or by adding reflectors that create scattering. However, both options are very expensive and may be prohibitive for consumer applications such as WLAN for home use.